Tactical Shotgun Build: Back Boring a Barrel and Lengthening a Forcing Cone to Increase Performance

Rifleshooter.com and Brownells modify a factory barrel with startling results

Was the juice worth the squeeze? Most definitely!

Our readers know we are huge fans of the Remington 870 and spend an inordinate amount of time tinkering with ours. As we write this, we’ve probably installed nearly every kind of ghost ring sight, stock, barrel, side saddle and magazine extension sold or imagined over the past 20- years. With the exception of cutting and crowning barrels, we’ve never actually addressed patterning performance. While traditional interchangeable choke tubes provide the shooter the ability to change the desired patterns as the situation mandates, the majority of tactical guns are left with fixed Improved Cylinder chokes. A select few may have their forcing cones extended and have their barrels opened up in a process called back boring.

The forcing cone is the tapered area just forward of the chamber that allows the shot an area in which to transition into the bore. As the shot then travels through the barrel, it passes through the choke at the end of the barrel. The difference between the bore diameter and the exit diameter of the muzzle, is known as the choke (see table below). For instance is the bore diameter is .729″ (standard for a 12 gauge) and the muzzle diameter is .689″, the barrel has a .040″ or full choke.

Constriction

(inches)

Name

Percentage of shot

in a 30 in circle

40 yd

.000

Cylinder

40

.005

Skeet

45

.010

Improved Cylinder

50

.015

Light Modified

.020

Modified

60

.025

Improved Modified

.030

Light Full

.035

Full

70

.045

Extra Full

.050

Super Full

So why lengthen a forcing cone or back bore?

By back boring a barrel, or increasing the diameter of the bore, you effectively increase the choke on a gun; which, in turn, tightens the pattern. This offers an advantage of increased range with shot shell and buckshot loads. A lengthened forcing cone provides a smoother transition for the shot leaving the chamber and results in less perceived recoil for the shooter. Since the transition is at a gentler angle, there is also less deformation of the shot pellets. An outstanding cutaway picture of a lengthened forcing cone can be found here.

No lathe? No problem.

While these processes are certainly easier with a large lathe, hand turning the reamers was surprisingly easy and provided outstanding results. Our barrel is a new, factory 18″ Remington barrel with a bead sight, part number 767-000-385WB. Before working on a barrel, it’s important to have a gunsmith inspect the wall thickness to ensure that there is enough material to safely remove some of it. Some barrels taper early and back boring or extending a forcing cone can provide disastrous results. To ensure this barrel was the appropriate thickness, we utilized two specialized tools, a Skeet shotgun bore gauge 843-200-126WB and a set of barrel calibers, 080-005-000WB.

We decided to begin with lengthening the forcing cone. We gathered a long Clymer forcing cone reamer 184-010-012WB, Do-Drill cutting oil 083-008-128, an acid brush and a large tap handle.

Forcing cone reamer and tap handle.Using rubber vice jaws, we secured the barrel vertically with a bucket beneath the muzzle to catch any excess cutting oil and shavings.You can’t use too much cutting oil when turning reamers by hand. We suggest you buy a gallon; it never goes bad and you can use it for all your cutting, tapping and drilling projects.We used an acid brush to apply a liberal coat of the Do-Drill oil.Carefully insert the reamer into the breech end of the barrel and turn. Every five turns we removed the reamer, cleaned off the shavings and reapplied the oil.Making progress, here you can see the first metal we cut from the barrel.The shavings begin to pile up.Almost there…..Success! The forcing cone is now lengthened.Material removed from lengthening the forcing cone.

If we weren’t back boring the barrel as well, we would then use a series of hones to smooth the tool marks from our newly cut forcing cone. Since we will be cutting this barrel some more, we will wait to hone the bore. Before moving to the next stage, we used an Otis Elite Cleaning Kit 668-000-077WB to remove any shavings and excess oil.

Back boring a barrel is more time consuming and labor intensive then lengthening a forcing cone. However, it is still enjoyable and provides excellent results. We were equipped with three reamers; Clymer .730″184-030-730WB, Clymer .735″ 184-030-735WB, and a Manson .740″ 513-030-740WB, as well as a 34″ Clymer back bore handle 184-030-012WB. Note, the Clymer reamers have 6-flutes and the Manson has 8. The Clymer reamers seemed slightly easier to turn by hand. Since the handle is long and one piece, we mounted the barrel horizontally in our vice.

Back Bore Reamers: .730″ (left), .735″ (center), and .740″ (right). Note the hole bored in the shaft to secure the reamer to an extension handle.The Reamer attached to the back bore handle with included roll pin. Use a roll pin holder and then a roll pin punch to set the pin.After coating the reamer with Do-Drill oil, we inserted it into the breech end, made five turns and removed it for cleaning and re-lubrication. Repeat this process until the reamer runs flush t0 the end of the muzzle.The metal keeps piling up. Make sure you clean the reamer often and keep it coated in oil.

Once we ran the .730″ reamer, we switched to the .735″ and finally the .740″. On our first attempt, we stopped running the reamers 2″ from the muzzle by making a witness mark on the reamer handle. Some sources suggested this would provide better choking performance. You will see this proved incorrect in testing.

We went back and bored the barrel with the tip of the reamer running flush to the muzzle this time and ended up with superior results. It is worth noting that these reamers are tapered, so running the end of the reamer to the muzzle still provides a smaller muzzle diameter. If you ran the reamer straight through you would create a cylinder bore.

After running the reamers, we thoroughly cleaned the barrel, removing all shavings and oil. Our camera was unable to capture the tooling marks on the inside wall of the barrel. While fairly smooth and probably usable, these marks would leave surfaces for fouling to accumulate and decrease performance. To address this we needed to hone the inside surface of the barrel.

We used a modern hone called a “Flex Hone”. These hones consist of small abrasive wheels bundled together at the end of a long flexible shaft. When matched with a special oil and chucked in a hand-held drill, these hones create an abrasive slurry that provides a mirror-bright shine to the bore. We used the 12-gauge flex hone kit 080-605-012WB (which includes a medium and fine grit bore hone as well as a fine chamber hone), medium forcing cone hone 080-606-512WB, and fine forcing cone hone 080-607-512WB. If you are using flex hones, make sure to read the instructions to ensure you don’t ruin them. They require special flex hone oil, which is included in the kit and they cannot exceed 750 RPM.

Once the honing was complete, we thoroughly cleaned the barrel and headed to the range. We tested 5-different barrels to compare performance. For our testing, we specifically used full velocity, non-plated buckshot. So we could see just how well our modifications worked, we opted not to use an ultra premium tactical load that is designed to shoot well from cylinder and improved cylinder barrels. For testing, we used Winchester Super-X 105-203-098WB, this is a decent load and is available in 15-round boxes from Brownells.

Conclusions:

We learned quite a bit on this project:

Shot pattern performance can be significantly increased with the use of hand tools and we are quite pleased with our results. While our barrel out performed the Vang Comp barrel at 15 and 20-yards, this was a limited test. Shot shells and buck shot in particular, can provide erratic results. We plan on testing these two barrels side by side more extensively in the future.

Buying the tooling will cost you a little more then having someone perform the operation on your barrel for you. However, if you have multiple guns or have a couple of friends; buying the tooling and modifying your own barrels will undoubtedly be significantly less expensive.

Cylinder barrels seem to perform surprisingly well. We actually noticed this on a few other factory cylinder choked guns we tested. We think that this may be attributed to better wad and buffer design and plan on investigating this further in the future.

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